The Pierre Auger Observatory is a hybrid detector for ultra-high energy cosmicrays. It combines a surface array to measure secondary particles at ground level together with a fluorescence detector to measure the development of airshowers in the atmosphere above the array. The fluorescence detector comprises 24 large telescopes specialized for measuring the nitrogen fluorescence caused by charged particles of cosmic ray airshowers. In this paper we describe the components of the fluorescence detector including its optical system, the design of the camera, the electronics, and the systems for relative and absolute calibration. We also discuss the operation and the monitoring of the detector. Finally, we evaluate the detector performance and precision of shower reconstructions.